用cte优化postgresql的一个案例

postgresql的查询计划有时候并不会按着我们建的索引走

例如  当我们创建了一张用户活跃表

create table td_user_active(usr_id int,usr_name text,upd_time timestamp with time zone);

insert into td_user_active select id,md5(now()::text),(current_date - interval '1000 days')::date+generate_series(1,id%1000) from generate_series(1,10000) id;

create index idx_td_user_active on td_user_active(id,upd_time desc);

 

现在我们查询下 每个用户最新的活跃时间

select usr_id,max(upd_time) from td_user_active group by usr_id;

时间花费了1702ms

explain analyze select usr_id,max(upd_time) from td_user_active group by usr_id;

HashAggregate  (cost=121607.57..121691.54 rows=8397 width=12) (actual time=2413.421..2416.425 rows=9990 loops=1)
   ->  Seq Scan on td_user_active  (cost=0.00..96632.71 rows=4994971 width=12) (actual time=0.330..779.988 rows=4995000 loops=1)
 Total runtime: 2417.269 ms
(3 rows)

Time: 2417.833 ms

该查询并没有走索引

下面就用cte优化

with recurive td_scan as (

(select usr_id,upd_time from td_user_active order by usr_id , upd_time desc limit 1)

union all

(select (select min(usr_id) from td_user_active where usr_id > td_scan.usr_id) as usr_id,

            (select max(upd_time) from td_user_active where usr_id=(select min(usr_id) from td_user_active where usr_id > td_scan.usr_id)) as upd_time from td_scan where td_scan.usr_id is not null)) select * from td_scan;

只用了501ms

explain analyze 。。。。。

 

 

 CTE Scan on td_scan  (cost=589.02..591.04 rows=101 width=12) (actual time=0.026..488.336 rows=9991 loops=1)
   CTE td_scan
     ->  Recursive Union  (cost=0.43..589.02 rows=101 width=12) (actual time=0.024..482.111 rows=9991 loops=1)
           ->  Limit  (cost=0.43..1.78 rows=1 width=12) (actual time=0.023..0.023 rows=1 loops=1)
                 ->  Index Only Scan using idx_td_user_active on td_user_active td_user_active_3  (cost=0.43..6749407.37 rows=4994971 width=12) (actua
l time=0.021..0.021 rows=1 loops=1)
                       Heap Fetches: 1
           ->  WorkTable Scan on td_scan td_scan_1  (cost=0.00..58.52 rows=10 width=4) (actual time=0.047..0.047 rows=1 loops=9991)
                 Filter: (usr_id IS NOT NULL)
                 Rows Removed by Filter: 0
                 SubPlan 2
                   ->  Result  (cost=1.79..1.80 rows=1 width=0) (actual time=0.022..0.022 rows=1 loops=9990)
                         InitPlan 1 (returns $2)
                           ->  Limit  (cost=0.43..1.79 rows=1 width=4) (actual time=0.021..0.021 rows=1 loops=9990)
                                 ->  Index Only Scan using idx_td_user_active on td_user_active  (cost=0.43..2260052.23 rows=1664990 width=4) (actual
time=0.020..0.020 rows=1 loops=9990)
                                       Index Cond: ((usr_id IS NOT NULL) AND (usr_id > td_scan_1.usr_id))
                                       Heap Fetches: 9989
                 SubPlan 6
                   ->  Result  (cost=4.02..4.03 rows=1 width=0) (actual time=0.023..0.023 rows=1 loops=9990)
                         InitPlan 4 (returns $5)
                           ->  Result  (cost=1.79..1.80 rows=1 width=0) (actual time=0.012..0.012 rows=1 loops=9990)
                                 InitPlan 3 (returns $4)
                                   ->  Limit  (cost=0.43..1.79 rows=1 width=4) (actual time=0.011..0.011 rows=1 loops=9990)
                                         ->  Index Only Scan using idx_td_user_active on td_user_active td_user_active_1  (cost=0.43..2260052.23 rows=
1664990 width=4) (actual time=0.010..0.010 rows=1 loops=9990)
                                               Index Cond: ((usr_id IS NOT NULL) AND (usr_id > td_scan_1.usr_id))
                                               Heap Fetches: 9989
                         InitPlan 5 (returns $6)
                           ->  Limit  (cost=0.43..2.22 rows=1 width=8) (actual time=0.022..0.022 rows=1 loops=9990)
                                 ->  Index Only Scan using idx_td_user_active on td_user_active td_user_active_2  (cost=0.43..1065.39 rows=595 width=8
) (actual time=0.009..0.009 rows=1 loops=9990)
                                       Index Cond: ((usr_id = $5) AND (upd_time IS NOT NULL))
                                       Heap Fetches: 9989
 Total runtime: 489.845 ms
(31 rows)

Time: 491.155 ms